A. Field of the Invention
This invention relates to color copier resistant documents. More particularly it relates to color copier resistant documents where machine copies made of such documents immediately denote themselves as invalid copies of the document. The indication of invalidity, which is clearly revealed on the copy, is substantially hidden on the original document.
B. Prior Art
In the past, copy-resistant documents such as those disclosed in U.S. Pat. Nos. 4,210,346, 4,227,719 and 4,227,720 were set forth as solutions to the problems created by the advent of color copiers. It is believed well known that documents reproduced by today's color copiers are so close in appearance to their originals that it takes an expert to differentiate between the two. In view of the great potential for harm that such excellent reproductions may have in the world's legal and financial communities, it becomes increasingly important to provide a simple system for differentiating copies of such documents from the original document.
Most of these earlier systems were composed of a single tone cancellation word fitted precisely into a significantly different background tone. Tone, in this instance, is intended to include both percent of area covered by the dots or marks and the distances between said dots or marks. In addition, the cancellation word VOID used was fairly large. For example, suggested sizes for the cancellation word VOID in these earlier systems was in the range of two (2) inches high and six (6) inches long. Thus, a check-sized document included only a single cancellation word. The letters used in the word VOID were composed of broad stroke geometric shapes. The cancellation word VOID and the background combination were then covered by another patterned screen which camouflaged the tonal difference between the word VOID and the background.
Though these previous systems have been useful they had several flaws. The use of the camouflage screen does help to make the word VOID less noticeable, but it is often not as well hidden as one would desire. Also, the large size of the word VOID limits the flexibility of placing illustrations, logo types, check formats, etc. on the document. Further, placing a custom design in the background tone to form a new background tone has produced a document with a satisfactorily hidden VOID word, but where the custom design covers the VOID word, it prevents reproduction of that portion of the VOID word which is covered. Naturally, this interference between the custom design and the cancellation word reduces the effectiveness of the hidden VOID when it appears on the copy of the original document.